Probably a tie between getting to dress up in costume (without looking out of place or going to a convention xD ), decorating, and Halloween parties that I plan on throwing someday.
Of course, snagging up some clearance sale priced candy after it's over is nice too.

I love dressing up - just being someone or something else for one night. Being able to step out of the usual and be something unexpected and completely different then myself even if only for one night and behind a mask.